It's either be velvet or ich with excess mucus from the fish. Either way you need to treat it with copper.
You also need to be very careful as not only are these fish butterfly fish but unless they've grow a lot, from the ones that were collected, they're very small and probably wouldn't take to good to the copper or other medicine.
I agree for the most part but if the fish do live through the copper treatment, I'd still prefer to see the addition of an antibacterial as it won't stress them more than the copper alone will and it can be safely and possibly effectively used with the copper.
Jon - are you going to try and find the Cupramine? Don't forget a test kit!. If you choose to add the antibacterial, I'd use Maracyn (for Saltwater). You're going to need ton of both if you are going to treat in your larger tank...